If you're running OS X, then you'll either need to install X Windows from the install CD that came with your Mac or you can try Seashore - based on the Gimp but a little more stripped down and built for Cocoa:

The last two options are completely free while the first is just a trial - if it's still available. Personally, I'm a fan of open source software but I still prefer Photoshop and Aperture for image processing/editing.